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Author Topic: Simple physics question  (Read 1161 times)
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Canuck
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« on: January 20, 2010, 12:22:16 PM »

So Im watching a snowboarding video and seeing these guys basically jumping off cliffs got me thinking about physics.  Now its been a while since highschool and I didnt pay so much attention anyways so I got to thinking, does a dropped object continue to accelerate infinitely? I mean assuming that it can keep on dropping that is.  An dropped object accelerates at 9.8m/s/s correct?  Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.  Anyways Im just curious what speeds these guys are going at when they finally reach the bottom.  Surely faster than something I would be able to land.
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dobberhd
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« Reply #1 on: January 20, 2010, 12:31:05 PM »

A dropped object will continue to accelerate until it reaches terminal velocity.  At terminal velocity the friction of the object with the air molecules (air resistance) is enough to stop the object from accelerating.
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TK-421
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« Reply #2 on: January 20, 2010, 12:31:50 PM »

Accelerations stops once terminal velocity is reached which for a person falling through air is about 120MPH.

Edit: Simulpost!  Mine has a link though.  smile
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TiLT
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« Reply #3 on: January 20, 2010, 12:36:24 PM »

There's also absolutely no difference in how fast two objects will accelerate. If you dropped a feather and an iron ball in a vacuum, they'd both hit the floor at the same time. Under normal conditions a feather falls much slower because of air resistance.
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Purge
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« Reply #4 on: January 20, 2010, 01:42:32 PM »

Quote from: TiLT on January 20, 2010, 12:36:24 PM

There's also absolutely no difference in how fast two objects will accelerate. If you dropped a feather and an iron ball in a vacuum, they'd both hit the floor at the same time. Under normal conditions a feather falls much slower because of air resistance.

Well duh. Both would be subject to the terminal velocity of the vacuum they were stuffed in.
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TiLT
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« Reply #5 on: January 20, 2010, 01:51:36 PM »

Terminal velocity wasn't a part of what I was talking about. I was responding to this:

Quote
Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.
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Shinjin
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« Reply #6 on: January 20, 2010, 01:52:17 PM »

Quote from: Purge on January 20, 2010, 01:42:32 PM

terminal velocity of the vacuum

Does not compute.
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dbt1949
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« Reply #7 on: January 20, 2010, 01:55:57 PM »

At my weight it's about 503 mph................then a thermonuclear explosion.
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Purge
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« Reply #8 on: January 20, 2010, 01:58:56 PM »

Quote from: Shinjin on January 20, 2010, 01:52:17 PM

Quote from: Purge on January 20, 2010, 01:42:32 PM

terminal velocity of the vacuum

Does not compute.

Quote
If you dropped a feather and an iron ball in a vacuum...

Then either both items hit the ground at the same time due to the fact that they are

A) both in the same vacuum
B) subject to the same terminal velocity as experienced by the "vacuum" (cleaner).

Explaining a joke is like an autopsy - neither the joke nor the patient survive it.
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dmd
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« Reply #9 on: January 20, 2010, 02:58:14 PM »

Quote from: Purge on January 20, 2010, 01:58:56 PM


Explaining a joke is like an autopsy - neither the joke nor the patient survive it.
There was a joke there?
Man survives autopsy.
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wonderpug
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hmm...


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« Reply #10 on: January 20, 2010, 06:53:49 PM »

Quote from: Canuck on January 20, 2010, 12:22:16 PM

Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.

Watch an astronaut drop a feather and a hammer on the moon.
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Bullwinkle
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« Reply #11 on: January 20, 2010, 07:27:21 PM »

Quote from: wonderpug on January 20, 2010, 06:53:49 PM

Quote from: Canuck on January 20, 2010, 12:22:16 PM

Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.

Watch an astronaut drop a feather and a hammer on the moon.

Schooled by Science!

Also there's this.

Spoiler for Hiden:
Schoolhouse Rocked by Science!
« Last Edit: January 20, 2010, 08:33:16 PM by Bullwinkle » Logged

That's like blaming owls because I suck at making analogies.
stimpy
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« Reply #12 on: January 20, 2010, 08:18:34 PM »

Quote from: wonderpug on January 20, 2010, 06:53:49 PM

Quote from: Canuck on January 20, 2010, 12:22:16 PM

Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.

Watch an astronaut drop a feather and a hammer on the moon.


Hey....wtf.
They just left them there?
Thats littering!!


We'll all be sorry when the Flying Hammer monsters attack.
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Purge
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« Reply #13 on: January 21, 2010, 02:26:46 AM »

Quote from: dmd on January 20, 2010, 02:58:14 PM

Quote from: Purge on January 20, 2010, 01:58:56 PM


Explaining a joke is like an autopsy - neither the joke nor the patient survive it.
There was a joke there?
Man survives autopsy.


Well, then he failed. It wasn't an autopsy; those are used to determine cause of death. Since he didn't die, it was just a curious look around his innards.
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Canuck
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« Reply #14 on: January 21, 2010, 04:08:06 AM »

So basically a feather and a bowling bowl start accelerating basically the same but because a feather is so light it almost immediately reaches its terminal velocity whereas the bowling ball would have a much higher terminal velocity.

Interesting stuff.
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Clanwolfer
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« Reply #15 on: January 21, 2010, 04:21:17 AM »

Quote from: Canuck on January 21, 2010, 04:08:06 AM

So basically a feather and a bowling bowl start accelerating basically the same but because a feather is so light it almost immediately reaches its terminal velocity whereas the bowling ball would have a much higher terminal velocity.

Interesting stuff.

Nope.  The feather and bowling ball example is misleading because the two have completely different aerodynamics.  The feather reaches terminal velocity because of its shape and distribution of mass, not just its light weight.
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Bullwinkle
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« Reply #16 on: January 21, 2010, 01:30:10 PM »

Quote from: Clanwolfer on January 21, 2010, 04:21:17 AM

Quote from: Canuck on January 21, 2010, 04:08:06 AM

So basically a feather and a bowling bowl start accelerating basically the same but because a feather is so light it almost immediately reaches its terminal velocity whereas the bowling ball would have a much higher terminal velocity.

Interesting stuff.

Nope.  The feather and bowling ball example is misleading because the two have completely different aerodynamics.  The feather reaches terminal velocity because of its shape and distribution of mass, not just its light weight.

Or not its light weight, at all.
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wonderpug
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« Reply #17 on: January 21, 2010, 05:04:39 PM »

A simple illustration of this is to drop a sheet of paper and a crumpled up sheet of paper at the same time.  Same mass; different terminal velocities.

You were right to some degree about there being a difference in gravity accelerating a feather and a bowling ball. It does take more force to accelerate the more massive bowling ball, but since the force of gravity goes up in proportion to the mass of an object, the force is increased exactly enough to make the bowling ball's acceleration due to gravity the same as the feather's.
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wonderpug
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« Reply #18 on: January 21, 2010, 05:10:24 PM »

Oh, and Bullwinkle the feather's low weight does play a part. A feather-shaped object made of lead will have a faster terminal velocity than a feather made out of a feather, despite being the same shape.  The air resistance upward force will be the same, but the downward force due to gravity (i.e., its weight) will be higher, and the downward force will exceed the upward force by a greater amount.
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« Reply #19 on: January 21, 2010, 05:21:40 PM »

Quote from: wonderpug on January 20, 2010, 06:53:49 PM

Quote from: Canuck on January 20, 2010, 12:22:16 PM

Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.

Watch an astronaut drop a feather and a hammer on the moon.

Why does the dust from their boots gut pulled down by gravity but not the flag? I know they are in a vacuum, but shouldn't the flag still be pulled toward the moon?
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McNutt
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« Reply #20 on: January 21, 2010, 05:25:42 PM »

I believe the flag had a support bar at the top so that it wouldn't drape.
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mytocles
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« Reply #21 on: January 21, 2010, 06:03:12 PM »

Damn you, Isaac Newton, and all your subsequent iterations - get off my lawn!   mad

Or, to translate (for at least me and dbt, and a fair number of the rest of you):  when I was in school there was no history!  Oh, wait - I meant physics... there was no physics...  Roll Eyes
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Mytocles (MY-toe-cleez)

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« Reply #22 on: January 21, 2010, 07:14:41 PM »

Quote from: Scraper on January 21, 2010, 05:21:40 PM

Quote from: wonderpug on January 20, 2010, 06:53:49 PM

Quote from: Canuck on January 20, 2010, 12:22:16 PM

Im pretty sure mass is invovled-a feather doesnt accelerate the same as a bowling ball surely.

Watch an astronaut drop a feather and a hammer on the moon.

Why does the dust from their boots gut pulled down by gravity but not the flag? I know they are in a vacuum, but shouldn't the flag still be pulled toward the moon?

Actually, that may also have to do with gravity.

The force of gravity is the displacement of space. The dust is more easily affected by the force of gravity due to the weak amount of force that influences it, whereas something like a flag (or person) requires more force.

Imagine a smooth bed with none of the imperfections of springs or blanket folds. Then place a 10-pin (big) bowling ball on it, so that it's at rest. Do this again, except with marbles of varying shapes and sizes (including ones as big as the 5pin - or small- bowling balls) scattered all over the surface.

The closest marbles, regardless of size, are going to be pulled closer to the big ball because the distance is short enough for the pull of gravity to draw them to it (and marginally, it to them). The marbles that are further away may be pulled in sooner since they are smaller, but as you travel outwards you'd find that the larger ones would stay (perhaps not in the bed example since the force of gravity is predominantly downwards from the Earth and not the bowling ball itself.

It is in this way that, should the expansion of our galaxy halt, that the force of gravity, over eons, drag everything back towards the centre.

That being said, the moon DOES have an atmosphere, it just isn't significant, and it is not a perfect vacuum and as such you can't expect the results to be correct.
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wonderpug
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« Reply #23 on: January 21, 2010, 07:24:43 PM »

wat
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mytocles
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« Reply #24 on: January 21, 2010, 07:56:01 PM »

Quote from: wonderpug on January 21, 2010, 07:24:43 PM

wat

Lol, really!  Who knew Albert was still alive - and his real name is Purge...    Tongue
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Shinjin
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« Reply #25 on: January 21, 2010, 08:14:22 PM »

Quote from: Purge on January 21, 2010, 07:14:41 PM

Actually, that may also have to do with gravity.

No



Quote from: McNutt on January 21, 2010, 05:25:42 PM

I believe the flag had a support bar at the top so that it wouldn't drape.

Yes.

They designed a flagpole with a horizontal bar allowing the flag to "fly" without the benefit of wind to overcome the effects of the moon's lack of an atmosphere
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Harkonis
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« Reply #26 on: January 21, 2010, 09:15:13 PM »

I was confused twice by Purge in the same thread.  glad to see I wasn't the only one.   icon_eek
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MonkeyFinger
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« Reply #27 on: January 21, 2010, 10:12:36 PM »

So much for the 'simple physics question'.  icon_wink
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pr0ner
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« Reply #28 on: January 21, 2010, 10:43:04 PM »

Quote from: Harkonis on January 21, 2010, 09:15:13 PM

I was confused twice by Purge in the same thread.  glad to see I wasn't the only one.   icon_eek

Hell, I'm a physicist, and my head hurts from reading what Purge has written.   icon_eek
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